chapter 14: mendel and the gene idea. heredity heredity= passing of traits from parent to offspring...
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Heredity
• Heredity= passing of traits from parent to offspring– Reproductive cells are only way traits are passed
to offspring– Changes in somatic cells will not be passed to next
generation• Genetics= scientific study of heredity
What genetic principles account for the passing of traits from parents to offspring?
1. The “blending” hypothesis is the idea that genetic material from the two parents blends together– Kind of like blue and yellow paint blend to make green
2. The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)– Heritable factors may be rearranged, but retain identity
from one generation to the next– This hypothesis can explain the reappearance of traits
after several generations
Gregor Mendel• Gregor Mendel, Austrian monk– Documented a particulate
mechanism in the 1800s by experiments with garden pea plants
• 1860s- Studied inheritance patterns in pea plants– Character= heritable feature with multiple variations– Trait= each type of variation for a character – At this time, chromosomes and genes had not been
discovered yet• Used the term heritable factors to represent what we call a gene
Gregor Mendel
• Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
• Advantages of pea plants for genetic study– Short-life cycle– There are many varieties with distinct characters
Gregor Mendel• Parents pass characters on to offspring– Pea plants= one character is flower color
• Parents can pass on different traits– Pea plants= purple or white flower color
Gregor Mendel
• Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
• Advantages of pea plants for genetic study– Short-life cycle– There are many varieties with distinct heritable
characters – Mating can be controlled• Each flower has sperm-producing organs (stamens) and
egg-producing organ (carpel)• Cross-pollination (fertilization between different plants)
involves dusting one plant with pollen from another
Gregor Mendel• Mendel chose to track
only those characters that occurred in two distinct alternative forms
• He also used varieties that were true-breeding in P generation• Produce offspring
of same variety when they self-pollinate
In a typical experiment, Mendel mated two different true-breeding varieties= hybridization
Gregor Mendel
• The hybrid offspring of the P generation are called the F1 generation
• When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced
Gregor Mendel
• F1 Generation: 100% purple flowers
• Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation
Gregor Mendel• Mendel reasoned that only the purple flower
factor was affecting flower color in the F1 hybrids
• The factor for white flowers was not diluted or destroyed because it reappeared in the F2
generation• Mendel called the purple flower color a
dominant trait and the white flower color a recessive trait
Pea Plant Experiments•7 Characters in Pea Plants•Crossed different varieties of each character
• Hybrids•Genetic Test Cross
• P generation• F1 generation• F2 generation
•Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
Mendel’s Hypotheses Based on 4 Concepts
1. There are alternate forms of genes (alleles) and these account for the variation of inherited characters.
2. Offspring inherit 2 alleles for each character, one from each parent.
3. When alleles are different, one allele determines appearance (dominant allele) and other has no effect (recessive allele).
4. Mendel’s Law of Segregation: Allele pairs separate during gamete production and sex cells carry one allele for each inherited character.
1. Alternate forms of genes (alleles) account for variation
• Alleles= alternative forms of genes• Flower Color: 2 Forms
2. Offspring inherit 2 alleles for each character, one from each parent.
• 1 chromosome with alleles for genes are inherited from mom and one from dad
• Alleles can be the same or different– Homozygous- alleles
same– Heterozygous- alleles
different
3. When alleles are different, dominant allele masks recessive allele
• Complete Dominance• Dominant Alleles (uppercase letters)– Determines appearance of offspring if present• Homozygous Dominant= PP
• Recessive Alleles (lowercase letters)– Masked by dominant allele– Only determines appearance if both alleles are
recessive – Homozygous recessive= pp
• Heterozygous: 1 dominant, 1 recessive= Pp
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more common in populations than recessive alleles– For example, one baby out of 400 in the United States
is born with extra fingers or toes• The allele for this unusual trait is dominant to the
allele for the more common trait of five digits per appendage
• In this example, the recessive allele is far more prevalent in the population than the dominant allele
Exceptions to the Rule• Incomplete Dominance:
Expression of recessive allele not completely masked in heterozygote– Snapdragon flower color
• Co-dominance: Two alleles are dominant and expressed in heterozygote– Example: Blood Type: AB
Red = RR
White = rr
Pink = Rr
4. Mendel’s Law of Segregation
• Allele pairs separate during gamete production and sex cells carry one allele for each inherited character
• Meiosis- haploid number of chromosomes• Fertilization- fusion of egg and sperm result in
diploid offspring– Genotype= alleles present in offspring for character
• Example: PP, pp, Pp
– Phenotype= physical appearance as a result of alleles• Purple flowers, white flowers
4. Mendel’s Law of Segregation
• Mendel derived the law of segregation by following a single character
• The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character
• A cross between such heterozygotes is called a monohybrid cross
Mendel’s Experimental Crosses
P Generation
PP pp
Cross homozygous dominant plant with homozygous recessive plant
Genotype and Phenotype
• Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition– Phenotype= physical appearance– Genotype= genetic makeup
• Flower color in pea plants:– PP and Pp plants have the same phenotype
(purple) but different genotypes
Heterozygotes
• Organisms with heterozygous genotype are considered “carriers” of the recessive allele– Not a true-breeding variety– Pea Plants Flower Color= Pp
• Recessive allele is present, but not “visible” in phenotype
• Can be passed on to next generation
Punnett Square• Used to predict potential traits in offspring– 4 possible combinations of alleles– Each parent creates 2 potential gametes 2 x 2= 4
• Need to know type of gametes produced by parent– Genotype= alleles for character– Each gamete has one allele
AA Aa
AA Aa
Pea Plants Experiment •Cross F1 generation to produce F2 generation•Phenotype: Purple Flowers•Genotype: Heterozygous•Cross Bb X Bb
•F2 generation: • 1 BB (purple)• 2 Bb (purple)• 1 bb (white)
Research in Genetics
• Testcross– Mating between unknown
genotype and homozygous recessive genotype
– Use Punnett Squares to predict offspring traits and compare to actual outcomes
Mendel’s Law of Independent Assortment
• Mendel identified his second law of inheritance by following two characters at the same time
• Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
Mendel’s Law of Independent Assortment
• Mendel’s Law of Independent Assortment– Genes inherited independently of each other– Inheritance of one character independent from
another character • This law applies only to genes on different
chromosomes or those far apart on the same chromosome– Genes located near each other on the same
chromosome tend to be inherited together
Genetic Inheritance
• Mendel’s laws of segregation and independent assortment reflect the rules of probability
• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss– In the same way, the alleles of one gene segregate
into gametes independently of another gene’s alleles
Genetic Inheritance
• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
• Probability in an F1 monohybrid cross can be determined using the multiplication rule
• Segregation in a heterozygous plant is like flipping a coin: – Each gamete has a ½ chance of carrying the dominant
allele and a ½ chance of carrying the recessive allele
Genetic InheritanceThe addition rule=
probability that any one of two (or more) exclusive events will occur is calculated by adding together their individual probabilities
Probability of heterozygote: ¼ + ¼ = ½
Probability of homozygote:¼ + ¼ = ½
Dihybrid Crosses• A dihybrid or other multicharacter cross is
equivalent to two or more independent monohybrid crosses occurring simultaneously
• Crosses with 2 characters of interest– RrYy x RrYy• In gametes, alleles for one character can be paired with
either allele for other character• For each parents gametes, R can be paired with Y or y • 4 possible gamete types for each parent
–RY, Ry, rY, ry
Exceptions to the Rule
• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
Exceptions to the Rule
• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
1. When alleles are not completely dominant or recessive
2. When a gene has more than two alleles3. When a gene produces multiple phenotypes
Exceptions to the Rule• Incomplete Dominance:
Expression of recessive allele not completely masked in heterozygote– Snapdragon flower color
Red = RR
White = rr
Pink = Rr
Exceptions to the Rule• Co-dominance: Two alleles are dominant and expressed
in heterozygote• This occurs in blood types, which also has more than 2
alleles– Four phenotypes: A, B, O, AB– A and B codominant, both are dominant to O
• Determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i
• The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate
• Enzyme encoded by the i allele adds neither
Pleiotropy
• Most genes have multiple phenotypic effects, a property called pleiotropy
• For example, pleiotropic alleles are responsible for the multiple symptoms of some hereditary diseases– Cystic fibrosis– Sickle-cell disease
Sickle-cell disease
• The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells– Causes cell to collapse and form sickle shape– Clogs blood vessels
• In homozygous individuals, all hemoglobin is abnormal (sickle-cell)
• Symptoms include physical weakness, pain, organ damage, and even paralysis
Sickle-Cell Disease• Heterozygotes= produce
both types of blood cells– Heterozygotes have sickle-cell
trait• Usually healthy but may suffer
some symptoms of disease
• Sickle-cell disease affects one out of 400 African-Americans – Unusual to have high
frequency of an allele with detrimental effects in homozygotes
Normal Blood Cell
Sickle-Cell
Sickle-Cell Disease• Reason: “Heterozygote
Advantage”– Less susceptible to the
malaria parasite, so there is an advantage to being heterozygous in areas where malaria is common
– Keeps allele in population despite detrimental effects of homozygous condition
Normal Blood Cell
Sickle-Cell
Epistasis• In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second locus
• In Labrador retrievers and many other mammals, coat color depends on two genes– One gene determines the pigment color (with
alleles for black or brown)– The other gene (with alleles
for color or no color) determines whether the pigment will be deposited in the hair
Polygenic Inheritance• Quantitative characters are those that vary in
the population along a continuum• Quantitative variation usually indicates
polygenic inheritance, an additive effect of two or more genes on a single phenotype
• Skin color in humans is an example of polygenic inheritance
Environmental Variation in Phenotypes
• Another exception arises when phenotype of a character depends on environment as well as genotype– Multifactorial characters: genetic and
environmental factors collectively influence phenotype
• Norm of reaction: phenotypic range of a genotype influenced by the environment– Usually broadest for polygenic characters
• Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Human Genetics
• Humans are not good subjects for genetic research – Generation time is too long– Parents produce relatively few offspring– Breeding experiments are unacceptable
• However, basic Mendelian genetics still serves as the foundation of human genetics
Pedigrees• Pedigrees can also be used to make predictions about
future offspring• We can use the multiplication and addition rules to
predict the probability of specific phenotypes• Many genetic disorders are inherited in a recessive
manner– Recessively inherited disorders show up only in individuals
homozygous for the allele• Carriers are heterozygous individuals with 1 copy of recessive allele
but are phenotypically normal• Most individuals with recessive disorders are born to carrier parents
• Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
Human Genetics• For a growing number of diseases, tests are available
that identify carriers and help define the odds of offspring having disease more accurately– Many diseases, such as heart disease, diabetes, alcoholism,
mental illnesses, and cancer have both genetic and environmental components, making predictions for multifactorial diseases difficult
• Now that the human genome has been described, research is focusing on identifying mutations in sequence and how they affect health– Advances in understanding the human genome will
potentially lead to new medicines, treatments, and preventive care
Current Genetic Research
• Breast Cancer– BRCA1 and BRCA2- human genes part of a group
of genes called tumor suppressors• Stabilize DNA and prevent uncontrolled growth
– Mutations in these genes increase risk of hereditary breast and ovarian cancers
– Mutations can be found in men and women and may increase risk of other types of cancers
– Blood test can detect genetic mutation• Not all mutations are harmful, many types are neutral
Current Genetic Research
• Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders– Individuals can evaluate chances of development
of genetic disorders present in close family members
– Potential parents with genetic disorders can determine the chances of passing genetic disorder on to offspring
Current Genetic Research• Punnett Square: Predicting the chance of
passing genetic disorder on to offspring– If a woman has a blood test and finds she has a
recessive mutation for a genetic disorder (heterozygous), what are the chances of her offspring having the mutation if her husband is homozygous dominant?
AA Aa
AA Aa
A a
A
A
Genotype Probability:50% AA (no mutation)50% Aa (mutation carrier)
Inbreeding
• In a large population, chance of mating with a relative is low because there are many choices– Most societies and cultures have restrictions against
marriages between close relatives• Sexual reproduction between non-related individuals
increases or maintains high genetic diversity• In a small population, chance of mating with a
relative is high because there are few choices– Increases frequency of deleterious recessive genotypes– Exposes rare deleterious mutations
Example: White Tigers• White coloration in Bengal
tigers– 1 in 15,000 wild births
• Homozygous recessive condition (aa)
• In 1951, one white male tiger was captured in the wild
• Most white tigers in zoos are descendants of this original tiger
Practice • Use a Punnett Square to find probabilities of
offspring from the following matings:1. White male (aa) x Orange female (AA)2. White male (aa) x Orange daughter (Aa)3. White male (aa) x White daughter (aa)
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Example: White Tigers
• Over time, repeated matings between close relatives to produce white tiger decreased genetic diversity and evidence of inbreeding depression was observed– Birth defects and low reproductive success– Weak immune system- more susceptible to disease– Poor eyesight and hearing– Weak bones
• In 2011, American Zoological Associationbanned breeding of white tigers
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